Three-phase slurry processes are known in the art. These processes are highly exothermic catalytic reactions. Basically, the three phases comprise a liquid phase in which solid catalyst particles are dispersed by a gas phase bubbling through the liquid phase. A typical three-phase slurry process is the Fischer-Tropsch hydrocarbon synthesis process in which synthesis gas, i.e., a mixture of hydrogen and carbon monoxide, is contacted with a catalyst dispersed in a hydrocarbon liquid. The catalyst used usually is a Group VIII metal catalyst supported on a catalyst carrier. Most of the hydrocarbons produced in this process are liquids under reaction conditions and need to be removed from the slurry reactor for further processing and upgrading to the desired end products.
In carrying out a three-phase slurry process, such as the Fischer-Tropsch hydrocarbon synthesis process, it is important to keep the catalyst and liquid inventory in the reactor vessel substantially constant. Thus, it is necessary to be able to separate liquid product from the catalyst and remove it from the reactor.
Various filtration schemes have been proposed to separate liquid, especially liquid hydrocarbon reaction products produced in a Fischer-Tropsch reaction, from the slurry in a three-phase slurry reactor. Some involve the use of filter elements located within the slurry reactor. Examples of these include U.S. Pat. No. 5,599,849, WO 2005/084791 A1, and US Patent Publications 2001/0039298 A1 and 2002/0128330 A1.
Other schemes involved the use of external filter systems. Examples of these include U.S. Pat. No. 5,770,629 and US Patent Publication 2003/0232894 A1.
Filtration systems that may be used within or outside of the slurry reactor are exemplified by U.S. Pat. No. 5,811,469 and US Patent Publication 2004/0235966 A1.
One problem with filtering systems such as described above is the build-up of catalyst particles on the filter which results in a decrease in the filter rate. Typically, the catalyst build-up is removed by periodic backflushing of the filter elements. Thus, for example, a surge vessel for backflushing is operably connected to one or more banks of filter elements via a common conduit. Valves may be provided, such as described in U.S. Pat. No. 5,599,849, for isolating individual banks of filter elements from the common conduit. This permits backflushing of individual banks of filter elements one at a time.
A key issue not addressed by the foregoing references is the need to protect downstream equipment from a potential mechanical failure of any of the many filter elements or piping connections located in the reactor. In a larger reactor, there may be thousands of filter elements, making it virtually impossible to guarantee complete system integrity for an extended run. The consequence of even a small leak in the filter system is a significant degradation of product quality, and loss of performance in downstream process equipment. One proposed approach to handle a filter element failure is to install a small filter screen in each primary filter element so that if an element fails, the solids plug the screen, thereby avoiding a large loss of catalyst into the downstream piping and equipment. However, this approach does not protect against a coupling or piping failure inside the reactor. In addition, the screen is in the flow path for the backwash so it may diminish the effectiveness of the backwash operation. Finally, the screen would not be accessible for maintenance during operation.
Thus, there remains a need for a filtration system for slurry reactors that improves the effectiveness of backflushing multiple filters and that provides failure detection and protection to downstream equipment from potential mechanical failure of a filter element or piping within the slurry reactor.